Technical Field
The technology relates to heteroepitaxial lateral overgrowth of semiconductor layers in vertically-confined geometries.
Discussion of the Related Art
Bulk silicon is a semiconductor material that is widely used for microfabrication of integrated circuits and other microstructure devices. Silicon is widely available and inexpensive, and the integrated electronics and microfabrication industries have developed many tools and processes for silicon-based technologies. Although silicon is widely used, other semiconductor materials can be desirable for certain microelectronic devices.
For example, gallium nitride (GaN) is a wide-bandgap semiconductor material that has useful applications in the areas of power electronics and light-emitting or detecting devices. Because of its wide bandgap, gallium nitride exhibits high breakdown voltages, an attractive property for power-electronic or high-voltage devices. When used for lighting applications, gallium nitride is capable of emitting or detecting short wavelength radiation in the blue and ultraviolet regions of the optical spectrum. Gallium nitride is widely used as the semiconductor material of choice for blue light emitting diodes (LEDs) and laser diodes (LDs). The advent of high brightness blue LEDs has revolutionized the lighting industry.
Although gallium nitride is an attractive material for certain applications, it can be a difficult or expensive material to form into large-area substrates for microfabrication purposes. One approach to forming large-area substrates of semiconductor materials other than silicon is to heteroepitaxially grow a layer of a selected semiconductor material onto the surface of a silicon substrate. FIG. 1 depicts a structure associated with conventional heteroepitaxy of gallium nitride on a substrate. According to some conventional methods, a seed layer 120 may be formed on a substrate 110. The seed layer 120 may be a crystalline material such as aluminum nitride (AlN). After the seed layer has been formed on the substrate, a layer of gallium nitride 130 may be heteroepitaxially grown from the seed layer. In some cases, the substrate may be a silicon substrate and oriented to have a crystallographic plane of (111) on its surface between the seed layer 120 and the substrate 110. The gallium nitride layer 130 may be grown by metalorganic chemical-vapor deposition (MOCVD), for example.
Conventionally, epitaxial growth of gallium nitride on a silicon substrate is difficult to achieve, because there is a significant lattice mismatch between the crystal lattice of the gallium nitride in the crystal lattice of the silicon substrate of about 17%. To mitigate the effects of the lattice mismatch, a seed layer 120 or multi-layers may be formed on the silicon substrate 110. Even with the use of a seed layer, the epitaxial grown gallium nitride may form with defects 135. The defects 135 may include stacking faults and dislocations as a well as other types of defects. The concentration of defects in the gallium nitride layer can be high when the gallium nitride is grown vertically from the substrate or seed layer. In some instances with thick gallium nitride layers, the defects can be in excess of 108 defects per centimeter squared (108 cm−2). For many integrated circuit applications, this level of defects is too high to provide satisfactory device performance.